[NETFILTER]: ctnetlink: check for listeners before sending expectation events

This patch uses nfnetlink_has_listeners to check for listeners in
userspace.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/ip_conntrack_netlink.c

@@ -1257,6 +1257,9 @@ static int ctnetlink_expect_event(struct notifier_block *this,
 	} else
 		return NOTIFY_DONE;
 
+	if (!nfnetlink_has_listeners(NFNLGRP_CONNTRACK_EXP_NEW))
+		return NOTIFY_DONE;
+
 	skb = alloc_skb(NLMSG_GOODSIZE, GFP_ATOMIC);
 	if (!skb)
 		return NOTIFY_DONE;

net/netfilter/nf_conntrack_netlink.c

@@ -1278,6 +1278,9 @@ static int ctnetlink_expect_event(struct notifier_block *this,
 	} else
 		return NOTIFY_DONE;
 
+	if (!nfnetlink_has_listeners(NFNLGRP_CONNTRACK_EXP_NEW))
+		return NOTIFY_DONE;
+
 	skb = alloc_skb(NLMSG_GOODSIZE, GFP_ATOMIC);
 	if (!skb)
 		return NOTIFY_DONE;